Crystallization properties of elastomeric polypropylene from alumina-supported tetraalkyl zirconium catalysts

An analysis of the crystallization properties of fractions of elastomeric polypropylene (ELPP) prepared with Al2O3-supported tetraalkyl zirconium catalyst is presented. A comparison with the polymorphic behavior of isotactic polypropylene (W) samples prepared with a single center homogeneous metallocene catalyst is also shown. The ELPP sample has been fractionated by extraction with boiling solvents. The irregular fraction insoluble in pentane and soluble in hexane crystallizes from the melt almost totally in the gamma form, whereas the more stereoregular fraction, insoluble in n-heptane, crystallizes mainly in the alpha form. The relative amount of gamma form crystallized from the melt is much lower than the one formed in samples of metallocene-made iPP samples with a similar average content of isotactic stereosequences. Since the gamma form crystallizes in chains having short regular isotactic sequences, these data indicate that in the fractions of the ELPP sample the regular isotactic sequences are longer than those present in chains of metallocene-made iPP. In particular, in the more irregular crystalline fractions of the ELPP sample the chains are characterized by a stereo-block microstructure, consisting in the presence of quite regular isotactic sequences alternating with irregular sequences, the latter containing most of the defects. The presence of stereoblocks allows crystallization of these highly irregular fractions, even in the presence of a very high content of defects, and accounts for the elastomeric properties shown by this sample. The whole ELPP sample is constituted mostly by amorphous ether soluble (40%) and pentane soluble (26%) fractions and shows elastic properties due to the high molecular weight of chains in all the fractions and the crystallization of isotactic sequences present in the stereoblocks. (C) 2004 Elsevier Ltd. All rights reserved.